The present invention relates to photoflash lamps and more particularly to high-voltage photoflash lamps.
A high-voltage flash lamp typically includes a glass envelope with a combustion-supporting gas and a quantity of filamentary, combustible material therein. A pair of electrically conductive lead wires is usually sealed in one end of the envelope and extend within the envelope. Medial portions of the extending ends of the lamp's conducting wires are located within a glass or ceramic bead. Primer material serves to bridge the portions of the ends which project through the bead. Flashing is accomplished by a firing pulse approaching a few thousand volts which is provided by a piezoelectric element. In another type of high-voltage lamp, the primer is located within an indentation in the bottom of the lamp and the conductive wires extend therein.
Understandably, it is highly desirous to prevent shred interference with the lamp's ignition. Shred interference can occur primarily in one of two ways: either by the shreds contacting and shorting the exposed portions of the lead wires within the envelope or by the shreds contacting and lying across the primer material surface.
In either case, the ignition voltage characteristics are altered, which in some instances can even prevent the lamp from firing. Shred interference can also reduce the firing voltage to the point that ignition is possible electrostatically. In situations wherein the lamp is used in circuitry containing several other lamps (e.g., sequential or random flash embodiments), an altered ignition voltage substantially reduces the lamp's compatibility with the desired circuit.
Various techniques for preventing shred interference with a lamp's ignition are illustrated. For example, in U.S. Pat. Nos. 3,884,615 and 3,685,947 a hollow glass bead is supported by the inner ends of the lead wires and primeer is put into the cavity of the bead to electrically connect the two lead wires. This construction is bulky and does not permit miniaturization of the flash lamp. In addition, the beads are costly, and their mass, which is relatively isolated from the bulb, interferes with rapid cool down and liquefaction of oxygen, thereby limiting machine speeds.
In U.S. Pat. Nos. 3,823,994 and 3,627,459, the inner ends of the lead wires are exposed inside a small length of glass tubing which is sealed into the press. A quantity of primer within the tube connects the two lead wires electrically. This construction is costly in that it requires small pieces of fabricated glass tubing.
U.S. Pat. No. 4,229,161 teaches the use of a device, such as a disc of mica, to isolate electrically the shreds in the lamp from the primer. The beaded construction is also shown. Both the bead and the mica disc add considerably to the cost of the lamp and the the difficulty of cooling the lamp and thereby pressuring the lamp with oxygen.
U.S. patent application Ser. No. 807,400 of Ronald E. Sindlinger for "Photoflash Lamp with Improved Primer", filed Dec. 10, 1985, and now U.S. Pat. No. 4,659,308 teaches the use of a primer including a combustible metal powder wherein at least part of the powder is in large particle form and solid electrically insulating beads to better insulate the internal leads of the lamp from the shred fill. The use of large particle form combustible metal powder with the electrically insulating beads essentially eliminates high voltage breakdown which occurs when electrically insulating beads are used with combustible metal powder all of which has an average particle size.
While the improved primer taught in Ser. No. 807,400 provides a photoflash lamp with improved reliability due to improved insulating of the internal leads from the shred fill and the essential elimination of high voltage breakdown, difficulty is encountered in applying the bead-containing primer to the lamp envelope. Primer is preferably added to the open-ended photoflash lamp envelope through a hollow needle dispense system. Such systems are preferred because they exclude air contact from the primer and prevent premature evaporation of the solvent from the primer. FIG. 1 is a schematic illustration of a portion of a hollow needle dispense system. In FIG. 1, the hollow needle portion 1 of a primer dispense device 3 is positioned within an open-ended lamp envelope 5. The primer dispense device contains a reservoir of primer (not shown). The hollow needle 1 of the primer dispense system delivers a predetermined amount of wet primer 7 (i.e., primer containing a volatilizable solvent) into the open-ended lamp 5. The primer then bridges the lead wires, 8 and 9, of the photoflash lamp.
When the needle dispense system is used with bead-containing primer, the beads become lodged in the hollow needle portion and, within a short period of time, totally block or clog the needle portion of the dispenser. Before the needle is totally blocked, wet primer not containing beads is forced between the lodged beads and dispensed into the lamp, resulting in non-uniform primer material in the lamps.
A photoflash lamp primer with high reliability and substantially consistent breakdown voltage and which can be applied to the lamp envelope with reduced difficulty and a photoflash lamp with improved manufacturability would represent an advancement in the photoflash art.